1. Field of the Invention
The present invention relates to a process for the production of high purity terephthalic acid and, in particular, to a process in which a crude terephthalic acid containing a relatively large amount of impurities, such as 4-carboxybenzaldehyde (denoted hereinafter as 4-CBA) etc., is treated by a purification step which permits a high yield of terephthalic acid.
2. Description of the Prior Art
Crude terephthalic acid product obtained by liquid phase oxidation of paradialkylbenzene contains in general, relatively large amount of impurities, mostly 4-CBA etc., so that it is not suitable for use in the production of polymer grade polyesters by direct reaction with a glycol due to its untolerable coloration. During the production of crude terephthalic acid, a mild oxidation condition is usually employed in the practice for decreasing the burning loss of a solvent, such as, acetic acid, etc., whereby the amount of impurities is further increased.
There have been proposed techniques for obtaining high purity terephthalic acid by purifying such crude terephthalic acid products containing impurities. Among them, those disclosed in Japanese Patent Publication Nos. 20820/1966, 23447/1968 and 23448/1968 have proposed processes based on the oxidation or hydrogenation treatment of crude terephthalic acid in a suspension in water or in a water/acetic acid solvent. Other proposals based on the treatment by oxidation or reduction of aqueous or water/acetic acid solution of crude terephthalic acid, which brings about higher catalytic efficiency than the above mentioned technique of treating the crude terephthalic acid in a suspension, are found in Japanese Patent Publication Nos. 21819/1967, 16860/1966, 46212/1977, 10051/1978, 32319/1981, 35174/1981, 35653/1981, 51373/1982, 51374/1982 and 51818/1982 as well as in Japanese Patent Application Lay-Open Nos. 1369/1973, 79635/1981 and 103136/1981.
Further proposals found in the prior art include those based on an after-oxidation of the resultant reaction mixture by heating it directly after the reaction, as proposed in Japanese Patent Publication No. 12695/1965; treatment of an aqueous solution of crude terephthalic acid with a catalyst of palladium or zinc, as proposed in Japanese Patent Publication Nos. 29131/1971, 3607/1972, 44213/1972, 13780/1974 and 33189/1974; and treatment of an aqueous alkaline solution of crude terephthalic acid with oxygen, as proposed in Japanese Patent Application Lay-Open No. 113738/1981.
All the techniques for purifying crude terephthalic acid proposed above are effective in some degree when the content of impurities is relatively low, nevertheless they are not always able to stand as satisfactory purification techniques, if the level of impurity content is high. In many cases, the color of crude terephthalic acid will not be improved though the content of 4-CBA, which is the principal impurity, can be reduced. In the prior art processes, the 4-CBA impurity is hardly converted into terephthalic acid and most of it will go to waste, so that no improvement in the yield of terephthalic acid will be expected and the amount of residue resulting from the purification process increases.
For example, according to the technique proposed by Japanese Patent Publication No. 46212/1977, it is described that an original content of 4-CBA of 2,300 ppm can be reduced to 0 ppm by a method in which a high temperature aqueous solution of crude terephthalic acid is treated in the presence of a definite catalyst with a gas mixture containing oxygen and terephthalic acid is then caused to crystallize. By this method, most of the 4-CBA content cannot be recovered by converting it into terephthalic acid and is lost out of the system in a form of benzoic acid due to decarbonylation. Therefore, no increase in the yield of terephthalic acid will be achieved, as the content of 4-CBA increases, though a purification thereof can be attained. Moreover, the color or hue of the so purified terephthalic acid product is not satisfactory. The substance causing deterioration of hue of the product cannot be removed sufficiently by the oxidation treatment and the amount thereof may even be increased on the conditions.
Also, by the purification method based on reduction treatment of aqueous solution of crude terephthalic acid with hydrogen as proposed in the Japanese Patent Publication No. 16860/1966, no increase in the yield of terephthalic acid can be accounted for, since in this case 4-CBA is converted into p-toluic acid.